Fibroblast growth factor 21 (FGF21) and its closest homologue FGF19 are members of the FGF superfamily. FGF21 signaling requires FGF-receptor (FGFR) isoforms and the membrane-bound coreceptor Klotho-beta (KLB) (Ogawa et al. Proc. Natl. Acad. Sci. USA 104(18): 7432-37 (2007); US2010/0184665). FGF19 has also been shown to signal through FGFR isoforms complexed with KLB (Wu et al. J. Biol. Chem. 282(40): 29069-29072 (2007)). Of the 7 primary isoforms of FGFR encoded by mammalian species (1b, 2b, 3b, 1c, 2c, 3c, and 4), only three isoforms, FGFR1c, 2c and 3c, can transduce signaling by both FGF19 and FGF21 when bound by coreceptor KLB, which is predominantly expressed in the liver, adipose tissue, and pancreas (Goetz and Mohammadi, Nature reviews. Molecular Cell Biology 14, 166-180 (2013)). Of these receptors, FGFR1c appears to play a predominant role in mediating the metabolic effect of FGF21. Without being bound to a particular theory, it is believed that FGF21 acts by inducing homodimerization of FGFR isoforms in the presence of the membrane-bound co-receptor KLB. Unlike other FGF ligands, FGF21 exhibits very low affinity to any individual FGFR. However, high affinity binding to KLB through the C-terminal tail region recruits FGF21 to the FGFR/KLB complex, allowing FGF21 to interact with FGFRs despite the low affinity to FGFRs alone.
FGF21 was identified as a potent disease-modifying protein agent to reverse obesity and type 2 diabetes in animal disease models (Kharitonenkov et al. J. Clin. Invest. 115(6): 1627-35 (2005)). Recombinant FGF21 has been shown to reduce hepatic lipids, improve insulin sensitivity, and normalize glycemic control in leptin-signaling-deficient (ob/ob or db/db) mice or high-fat diet (HFD)-fed mice. Reduction in blood glucose and improvements in various cardiovascular risk factors have also been observed in obese and diabetic rhesus monkeys treated daily with recombinant FGF21. FGF21 and FGF19 have both been shown to activate the thermogenic function of uncoupling protein 1 (UCP1)-positive adipose tissues (brown and beige adipose tissues; BAT) in obese rodents (Fu et al., Endocrinology 145, 2594-2603 (2004); Coskun et al., Endocrinology 149, 6018-6027 (2008); Fisher et al., Genes & Development 26, 271-281 (2012)).
Although clinical applications of recombinant FGF21 or FGF19 analogs are currently being tested for the treatment of metabolic disease, their development for therapeutic intervention has proven challenging. For example, the serum half-life of FGF21, hours in non-human primates, is too short for practical clinical application and the remaining FGF21 protein in circulation can be rapidly inactivated by proteolytic cleavage. Efforts have been made to improve these properties through protein engineering, but such modifications could increase immunogenicity and other modification-specific adverse effects. Another significant challenge is a possibility of long-term adverse effects from chronic FGF21-mediated therapy. For example, FGF21 has been reported to induce hepatic growth hormone resistance via induction of SOCS2, an inhibitor of growth hormone receptor signaling (Inagaki et al., Cell Metab. 8: 77-83 (2008)). In humans, growth hormone resistance or deficiency is associated with low bone mass in children and adults and transgenic overexpression of FGF21 or two weeks treatment of mice with recombinant FGF21 leads to a dramatic loss of bone mineral density. It has not yet been demonstrated that the bone-related adverse effects of FGF21 can be de-linked from the beneficial metabolic effects. Further, transgenic overproduction of FGF19 can lead to hepatocellular carcinogenesis via activation of FGF Receptor (FGFR) 4 (Fu et al., Endocrinology 145, 2594-2603 (2004); Tomlinson et al., Endocrinology 143, 1741-1747 (2002); French et al., PLoS One 7, e36713 (2012)).
Recombinant monoclonal antibodies (Abs) can act as a powerful therapeutic modality as they can provide excellent target selectivity, pharmacokinetic profile, and other properties important for a pharmaceutical agent (Chan and Carter, Nature reviews. Immunology 10, 301-316 (2010)). For example, an antibody antagonist specific for FGFR1c was reported to induce weight loss in mice and monkeys (WO2005/037235) and agonistic antibody-mediated selective activation of FGFR1c is sufficient to recapitulate the insulin sensitization by FGF21 in diabetic mice (WO2012/158704; Wu et al. Science Translational Med. 3(113): 1-10 (2011)). Antibodies that bind to the KLB/FGFR1c complex have been proposed as activators/therapeutic agents (U.S. Pat. No. 7,537,903; WO2011/071783; WO2012/158704). Others have investigated two alternative approaches to selectively activate the FGFR1c/KLB complex, such as a high affinity anti-KLB antibody called mimAb1 (Foltz et al. Sci. Transl. Med. 4: 162ra153 (2012)) and bispecific anti-FGFR1/KLB Avimer polypeptide C3201 linked to human serum albumin (HSA) (U.S. Pat. No. 8,372,952).
Given the significant role for FGF19 and FGF21 in glucose metabolism, there remains a need in the art for the development of therapeutic molecules and methods to modulate FGF19 or FGF21-mediated activities.